This invention relates to a method for making the heat transfer surfaces of cooling water systems and boilers resistant to the formation of scale deposits and fouling. More particularly the invention relates to a method for the chemiphoretic coating of metallic heat transfer surfaces with a polymer latex, thereby rendering the surfaces resistant to the deposition of scale and fouling, and to metallic surfaces coated by the process of this invention.
In heat exchange equipment, the formation of scale and fouling on surfaces in contact with aqueous fluids lowers the heat transfer efficiency of the surface and can cause overheating and damage. Scaling is caused by crystallization and/or precipitation of salts, mainly calcium carbonate, to form a hard adherent layer on the surface. Such scale may be controlled by pretreatment to remove scale-forming constituents or by increasing or broadening the solubility of scale-forming salts through use of dispersants, often coupled with blowdown procedures to remove accumulated sludges and to lower the concentration of dissolved solids. Although many such prior art methods exist which markedly reduce the scaling rate, these do not completely eliminate the problem and with time, scaling becomes sufficient to reduce efficiency and require cleaning and/or replacement of the heat transfer surfaces.
Heat transfer surfaces exposed to aqueous fluids are made markedly resistant to the deposition of adherent scale by coating with plasma polymerized fluoroethylene monomer such as tetrafluoroethylene, as is shown, for example, in U.S. Pat. No. 4,125,152. Plasma polymerization provides poly(tetrafluoroethylene) PTFE coatings on substrates which, being uniform and very thin, do not significantly affect heat transfer properties of heat transfer surfaces. Although the resulting surfaces are usefully resistant to scale deposition and fouling, the difficulty of forming such coatings in areas having restricted access, such as, for example, upon the inner surfaces of heat exchange tubing and the like, mitigates against wide-spread applicability for use in a variety of commonly employed heat exchange devices.
A method for the deposition of uniform coatings on ferriferrous surfaces from polymer latices was described in 1971 by Steinbrecher, et al, U.S. Pat. No. 3,585,084. The process, now termed chemiphoresis, depends upon the etching and dissolution of ferrous ions from steel surfaces by the acidic latex compositions. The resulting high concentration of ions at the surface tends to destabilize the latex next to the steel surface, causing film deposition to occur. More recently, T. Nishida, et al in German Patent Application OS 2,409,987, published Sept. 26, 1974, disclosed a chemiphoretic process for latex deposition on a variety of metallic substrates including copper and aluminum. According to Nishida et al, the Steinbrecher process had application only to ferriferrous metal surfaces by virture of the fact that such materials as lead, copper and the like were etched too slowly by the acidic latex, thus producing only a slow buildup of metallic ions which in turn was insufficient to cause adequate film deposition on non-ferrous surfaces. Nishida's method overcomes the limitations of the Steinbrecher coating process by addition of metallic ions to the latex composition at a concentration near the critical limit value. The latex compositions of Nishida et al further contain an acid capable of etching the metal surface, thereby providing a sufficient increase in metallic ion concentration at the surface to destabilize the latex and cause film deposition on the surface. Nishida et al note that the inclusion of oxidizing agents such as hydrogen peroxide also may be useful in the practice of that process to speed the deposition and consequently increase the thickness of the coating. The Nishida et al process and the Steinbrecher et al process require acidic conditions and the coating latex compositions are adjusted to a pH in the range of from 1.6 to 5.0. Above the specified range, little or no coating takes place.
Many desirable coating resin latices are commercially available that are not stable under acidic conditions and cannot therefore be used in these prior art processes. A method for depositing such coating resins from a neutral or alkaline latex upon metallic substrates and particularly upon copper and aluminum substrates would greatly increase the range of coating resins available to the industry for use with such substrates and would thus be a significant advance in the coating art.